The present invention relates to a novel process for the synthesis of zeolites of the ferrisilicate type called ferrizeosilites, the products obtained by this process.
The zeolites are crystalline tectosilicates. Their three dimensional structure is constructed by an assembly of TO.sub.4 tetrahedra, placing their tops in common, two different tetrahedra only having one oxygen in common. In zeolites of the aluminosilicate type, which are the most common, T represents tetravalent silicon as well as trivalent aluminum. The cavities and channels of molecular dimensions, of the covalent framework receive the cations compensating for the charge deficit associated with the presence of the trivalent aluminum in the tetrahedra. Some rare zeolites are also known wherein the silicon is replaced by tetravalent germanium. Similarly, trivalent elements like gallium and more rarely boron or divalent elements like beryllium may be substituted for the aluminum.
Generally, the composition of the zeolites may be represented by the crude formula M.sub.2/n O, Y.sub.2 O.sub.3, xZO.sub.2 in the dehydrated and calcined state. Z and Y represent respectively the tetravalent and trivalent elements of the tetrahedra TO.sub.4 ; M represents an electro-positive element of valency n, such as an alkali or alkaline-earth; x can vary from 2 to theoretically infinity in which case the zeolite is a silica.
Each type of zeolite has a distinct porous structure. The variation of the sizes and shapes of the pores from one type to another results in a change in the absorbance properties. Only molecules of certain sizes and shapes can enter the pores of a particular zeolite. The chemical composition with, in particular, the nature of the exchangeable compensation cations, is also an important factor taking part in the selectivity of the adsorption and especially the catalytic properties of these products.
Due to their particular characteristics (molecular sieves and cation exchangers, the zeolites are used both in adsorption and in catalysis. Among the uses in adsorption may be mentioned the purification of gases, the separation of hydrocarbons; in catalysis several important processes use zeolites as catalysts: catalytic cracking, hydrocracking, isomerisation etc.
Although numerous zeolites of the aluminosilicate type exist in nature, research for products having novel properties has led in the course of recent years to the synthesis of a great variety of these aluminosilicates with a zeolitic structure.
In addition, several patents claim the partial or even total substitution of the aluminum atoms of the crystalline framework by elements of oxidation degree III, (boron, gallium, iron . . . ). After replacement of the compensation cations by protons by techniques known in the prior art, acid solids are obtained in which the strength of the sites will vary according to the nature of the one or more elements (boron, gallium, iron . . . ) which have been substituted for silicon within the framework. Thus, for example, zeolites containing silicon and iron in their crystalline framework have acid properties different from those of zeolites of the same crystalline structure containing silicon and aluminum in their framework.
By way of example, in the case of substitution of Al by FeIII, see German patent DE No. 2831611, European patents EP 813532, 884422 and 115031.
In addition, the prior art may be illustrated by the patents EP-A-160,136, EP-A-30,751 and CA-A-1.197.498.
Generally, the zeolites are prepared by hydrothermal crystallization of reaction mixtures containing alkaline or alkaline-earth hydroxide sources, silica, and oxides or salts of elements like aluminum which can replace the silicon in the tetrahedra.
The addition to the reaction mixture of a generally organic structural member, such as an amine or a quaternary ammonium salt, is often necessary for the formation of said zeolite. The pH of the whole of the preparation is basic and generally higher than 10. It is acknowledged that the concentration of OH.sup.- ions facilitates the crystallization of the zeolite by ensuring the dissolution of the silica sources, and, possibly of amphoteric oxides like alumina, as well as the transfer of the soluble species thus obtained to the zeolite being formed.
This method of synthesis of the zeolites has numerous drawbacks particularly if it is desired to replace the aluminum by iron. In fact, in a basic medium, the majority of the zeolites synthesized are metastable and there is a risk of the appearance, in the course of the preparation, of more stable but undesired solid phases, as well as the precipitation of ferric hydroxide. This difficulty only increases when the amounts prepared increase, that is to say on the industrial scale.
Besides, these zeolites which are metastable in the basic reaction medium are only obtained from a high supersaturation of active species in the medium, which causes rapid nucleation and leads to zeolite crystals of small size, the average size of these crystals being in the range of a micrometer. The formation of crystals of larger size is hence difficult. In certain applications of ion exchange, adsorption or catalysis, it would be interesting to be able to work with crystals of large size which, for example, would permit the conditioning of the zeolites by agglomeration, with all the drawbacks that this brings, being avoided.
Numerous applications, in particular in acid catalysis, require zeolites in a protonized form and completely freed from their alkaline or alkaline-earth compensation cation introduced during the synthesis. It is possible here to arrive at this by repeated and long ion exchange processes with NH.sub.4.sup.+ cations followed by calcination to decompose them into H.sup.+ cations. This ion exchange step could be eliminated if it were possible to replace entirely the alkali metal or alkaline-earth metal cations by NH.sub.4.sup.+ cations during the synthesis. Now, this is not possible when the pH substantially exceeds 10, NH.sub.4.sup.+ being converted under these conditions into NH.sub.3. In addition, synthesis carried out at pH's where the NH.sub.4.sup.+ cation is stable are difficult and time consuming on account of the low solubility of the sources of silica at these low pH's.